The present invention relates generally to an optical moisture sensor for detecting moisture on the surface of a transparent material, and more particularly, to a servo circuit for self adjusting the moisture sensor for use with different transparent materials having different transmittances.
The accumulation of moisture on transparent materials, such as glass or plexiglass, can obstruct a person's view through the material. Motor vehicles have long been equipped with motor-driven windshield wipers for clearing the moisture from the external surface of the windshield, at least within the driver's field of vision, and generally over a larger area so as to enhance one's vision through the windshield.
In most vehicles today, the windshield wiper system includes multi-position or variable speed switches which allow the driver to select a wide, if not an infinitely variable, range of speeds to suit conditions. Wiper controls are manually operated and typically include a delay feature whereby the wipers operate intermittently at selected time delay intervals.
Wiper control systems have recently been developed which include a moisture sensor mounted on one of the vehicle windows to automatically activate the wiper motor when moisture is deposited upon the surface of the window. The wiper control system including the moisture sensor are most typically mounted on the windshield, although the system may be mounted on the rear window or any other glass surface intended to be cleared of moisture. Such wiper control systems free the driver from the inconvenience of frequently adjusting the wiper speed as the driving conditions change.
Wiper control systems have used a number of different technologies to sense the moisture conditions encountered by a vehicle, including conductive, capacitive, piezoelectric, and optical sensors. Optical sensors operate upon the principle that a light beam is diffused or deflected from its normal path by the presence of moisture on the exterior surface of the windshield. The systems which employ optical sensors have the singular advantage that the means of sensing disturbances in an optical path is directly related to the phenomena observed by the driver (i.e., disturbances in the optical path that affords the driver vision). McCumber et al. (U.S. Pat. No. 4,620,141) disclose an optical moisture sensor which triggers a sweep of the wiper blades in response to the presence of water droplets on the exterior surface of a windshield.
In typical optical moisture sensors, a light signal from an emitter is directed into the windshield at an angle of approximately forty-five degrees with respect to the windshield. The light signal is then reflected by the outer surface of the windshield at approximately a forty-five degree angle and the reflected signal is directed into a detector. The presence of moisture on the surface of the windshield affects the reflection of the light signal at the outer surface of the windshield resulting in a reflected signal having a lower amplitude. The detector receives the reflected signal and produces an output signal which indicates the change in amplitude of the reflected emitter signal. The detector output signal is electronically processed by the moisture sensor which determines when to actuate the windshield wipers.
Moisture sensors, especially those used in automobiles, typically operate under a wide range of temperatures which can affect the electrical values of the electronic components. In addition, the electrical values may change as the electronic components age. These changes in the values of the electrical components can affect the strength of the emitter signal, which changes the detector output and ultimately affects the performance of the moisture sensor. It is desirable for the moisture sensor to compensate for these changes in component values and continue to operate within the desired specifications.
Additionally, the moisture sensor may be used on different automobiles having different glass transmittances. The transmittance of the glass determines the amount of light which will pass through the glass. Therefore, the transmittance of the glass affects the strength of the reflected emitter signal reaching the detector. For example, modern solar-control windshields, such as windshields sold under the trademark "EZ-KOOL" commercially available from Libbey-Owens-Ford Co., absorb much of the infrared rays used by many optical moisture sensors drastically reducing the strength of the reflected emitter signal. It is desirable to adjust the intensity of the emitter signal in relation to the transmittance of the glass to achieve a reflected signal having a predetermined amplitude regardless of the glass transmittance.
It is known to provide automatic intensity control for moisture sensors (see Mangler, et al. U.S. Pat. No. 5,436,541). Teder (U.S. Pat. No. 5,059,877) teaches an automatic intensity control circuit which uses an analog linear servo. The Teder '877 patent teaches using an analog integrating device for adjusting the intensity of the emitter signal such that the amplitude of the detector output signal matches a threshold signal. The circuit is a linear circuit and thus the intensity of the emitter signal is changed by an amount which is proportional to the difference between the amplitude of the detector output signal and the threshold. An analog automatic intensity control is taught by Mangler, et al in U.S. Pat. No. 5,436,541. While the analog circuit performs adequately, it is desirable to provide an automatic intensity control circuit which is less expensive and which can be more effectively adapted to digital control.
Hasch et al. (U.S. Pat. No. 5,225,669) teaches using digital components to implement an automatic intensity control unit for moisture sensors. The Hasch device attempts to maintain the amplitude of the detector output signal within a target range. The target range includes an upper threshold defined by a first comparator and a lower threshold defined by a second comparator. The intensity of the emitter signal is changed when the amplitude of the detector output signal falls outside of the target range. When the amplitude of the detector output signal stays within the target range, the intensity of the emitter signal is not changed. An automatic intensity control unit as practiced by Hasch requires electrical components to establish two separate thresholds. Furthermore, the system permits the detector output current to fall anywhere within the prescribed range, rather than holding it to a single precise value. This introduces the potential for variation of sensor sensitivity. It is desirable to reduce the components necessary to implement an automatic intensity control unit while improving the sensitivity of the moisture sensor.